In the compressor portion of an aircraft gas turbine engine, atmospheric air is compressed to 10-25 times atmospheric pressure, and adiabatically heated to 800.degree.-1250.degree. F. in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, on the order of 3000.degree. F. These hot gases pass through the turbine, where rotating turbine wheels extract energy to drive the fan and compressor of the engine, and the exhaust system, where the gases supply thrust to propel the aircraft. In idealized engine operation, under stoichiometric conditions, the combustion process would be complete, and only the carbon dioxide and water vapor produced in combustion, plus nitrogen and minor components of atmospheric air, would be exhausted to the atmosphere by the engine. In reality, significant amounts of carbon monoxide (CO) and unburned hydrocarbons (UHC) are discharged into the atmosphere, particularly when the engine is operated at low speed. Such emissions are undesirable, for they pollute the environment and they represent a waste of fuel.
Operating parameters vary widely during operation of an aircraft gas turbine engine, from idling to developing very high thrust for take-off and in-flight maneuvering. As a result of these and other situations, conditions in the combustor also vary widely, possibly resulting in flame instability or even flame-out. The latter term refers to quenching the flame in the combustor due to disruptions in the gas flow within the combustor. Designers of aircraft gas turbine engines have devoted considerable effort toward minimizing flame instability and/or the likelihood of flame-out, and combustors are typically designed to operate under conditions where flame-out is not likely. Relighting an engine after flame-out during flight is essential to continued engine operation, but it is not always easily achieved, and thus designers seek to avoid flame-out.
There are several approaches to controlling flame instability and flame-out, including the use of catalytic substances. For example, Christensen (U.S. Pat. No. 2,829,494) teaches the use of a catalytic target positioned within a combustor immediately downstream from the point where fuel is injected. Vdoviak (U.S. Pat. No. 3,032,991) teaches the use of a multi-layer pad of fine-wire catalytic screen to sustain combustion in an augmentor, or afterburner, attached to the downstream end of an aircraft gas turbine engine. Teague (U.S. Pat. No. 3,113,885) teaches the use of flame holders which are, in effect, probes coated with catalytic material that extend into the combustor. Pfefferle et al. (U.S. Pat. No. 4,603,547) teach the coating of portions of the upstream end of a combustor with a thermal barrier coating, to which a catalytic coating, preferably a noble metal, is applied. The disclosure of each of these patents is incorporated herein by reference. Another common approach to the problem is the widespread use of electrical igniters in combustors.
Certainly these approaches to the related problems of maintaining flame stability and preventing flame-out are important advances in the gas turbine engine art, but these teachings do not address another problem, namely, the emission of CO and UHC during operation of the engine. The need to reduce such emissions to prevent degradation of the environment, is apparent. The present invention is directed specifically to the reduction of such undesirable emissions.
Other uses of catalytic materials are illustrated in patents by Pfefferle and Angwin et al. In U.S. Pat. No. 3,928,961, Pfefferle teaches the use of an externally heated bed of catalytic material to start a thermal system such as a gas turbine engine, reducing the emission of CO and UHC during the starting process. In U.S. Pat. No. 4,065,917, Pfefferle teaches the use of a bed of catalytic material in a turbomachine to achieve adiabatic combustion at temperatures lower than typically encountered in such machines, thereby reducing emissions of oxides of nitrogen. Angwin et al. (U.S. Pat. No. 4,337,028) teach the use of monolithic ceramic materials containing catalytic materials mixed therein, specifically in contrast to the use of catalytic coatings. The disclosures of these patents are also incorporated herein by reference. However, these patents address other aspects of catalysts of combustion, that are distinct from the present invention.
It is an object of the present invention to provide means for reducing the emission of CO and UHC by a gas turbine engine.
It is also an object of the present invention to provide such means while, at the same time, increasing the efficiency of such an engine.
It is a further object of the present invention to provide such means without adversely affecting the flow of gases through the engine, such as by avoiding the use of beds, screens, honeycombs and the like, made of catalytic material, which may be used to accomplish some of the objects of the present invention, but which interfere with the flow of gases through the engine.
It is another object of the present invention to provide such means without appreciably increasing the weight of the engine; for the size and weight of catalytic converter beds represent a penalty that is generally deemed unacceptable on a airborne gas turbine engine.
It is yet another object of the present invention to provide such means in a way as to minimize the impact on the design of such gas turbine engines and fabrication of components therefor.
It is still another object of the present invention that reducing the emission of CO and UHC by a gas turbine engine may be achieved by retrofitting existing engines, such as by simply replacing components of these engines during routine maintenance work.